Radar Products

Spectrum Width -- a measure of the amount of variability in the radial velocity data.

Dual Pol Products

Correlation Coefficient (CC) - measure of how similarly the horizontally and vertically polarized pulses are behaving. Values 0.2 to 1.05 with Units. CC is referred to as PHV in the professional literature. Values of CC < .8 suggest non-meteorological targets such as birds, insects, etc. Moderate values of CC (0.8 to .97) indicate non-uniform meteorological targets such as hail, melting snow, etc. High values of CC (>.97) indicates uniform meteorological targets such as rain, snow, etc. Deviations from the ranges above may occur as the distance from the radar site increase, a mix of hydrometeors is present, the beam encounters the 'melting layer', range folding or non-uniform Beam Filling (NBF). CC is very good at identifying the 'melting layer'.

Stratiform rain or snow generally yield CC values from 0.97 to 1.00. Hydrometeors with complex shapes or a mixture of hydrometeors will have CC values from 0.85 to 0.95. Any form of water-coated ice has rather low CC values. For small, dry hail and dry graupel, CC is close to 1.0. The melting layer, where falling snow begins to melt and become water-coated, produces CC values between 0.65 and 0.95. Ground clutter, biological targets, chaff, and wind-blown debris have CC values at the lower end of the scale, typically less than 0.7.

Differential Reflectivity (ZDR) - Differential reflectivity is just the difference between the reflectivity factor from horizontally polarized pulses and that from vertically polarized pulses. (The definition of ZDR is ZDR = ZHZV). The range of values is from −7.9 to +7.9 dB.

Postive Values of ZDR: Indicates that more horizontal power return than vertical power return meaning the dominant hydrometeors are larger in the horizontal (rain drops).

Negative Values of ZDR: Indicates that more vertical power return than horizontal power return meaning the dominant hydrometeors are larger in the vertical (vertically oriented crystals or conical graupel).

Near Zero Values of ZDR: indicate that the horizontal and vertical power returns are similar in size.

The ZDR chart shows typical ZDR values for various targets. As hinted above, ZDR for nearly spherical targets is close to zero, but values increase with drop size as the drops become more oblate. Very large hail sometimes produces negative ZDR values. Small hail, not water-coated, tends to tumble randomly and thus, to the radar, appears to be spherical (ZDR near zero). If hail becomes coated with liquid, it appears to the radar as a giant raindrop, and ZDR increases. Similarly, if rain and hail are in the same sample volume, the ZDR increases. Graupel, often the embryo for hail, is a low-density white pellet that is filled with air bubbles. Dry graupel often has a conical shape and may be taller than it is wide, and so yields negative ZDR values. If graupel becomes water-coated, its ZDR value increases. Dry snow generally has ZDR values close to zero, but if it begins to melt, the ZDR increases markedly. Considering ice crystals, if many of them are interlocked in a flake, the ZDR is close to zero. Individual crystals assume many shapes, none even remotely spherical. Needles, columns, and plates tend to fall with their long axes horizontal, and so their ZDR values can be large. Ground clutter refers to objects on the ground seen by the radar, either because they are tall (buildings) or because the temperature stratification in the lower atmosphere bends the radar beam (anomalous propagation–AP) so that the radar sees more of the ground than usual. Biological targets include birds, insects, and bats. Chaff refers to thin strips or wires of aluminum sometimes dropped near or inside of clouds to track air flows and sometimes for military purposes. Debris refers to objects lofted from the ground by high winds, including tornadoes.


Hydrometeor Classification (HCA) -

Specific Differential Phase (KDP) - The range derivative of the differential phase shift between the horizontal and vertical pulse phases. Values are -2 to 10, Units are degrees per kilometer (deg/km).


Melting Layer

The melting layer algorythm is searching for wet snow using Z (between 30 and 47 dBZ), ZDR (between .8 and 2.2 zDB) and CC greater than .85.

On radar imagery the melting layer is plotted using concentric rings:

  • Inner Ring: top of beam enters the melting layer
  • Second Ring: center of beam enters the melting layer
  • Third Ring: center of beam exits the melting layer
  • Fourth Ring: bottom of beam exits the melting layer